A technique for forming a thin film transistor (TFT) by using a semiconductor thin film formed over a substrate having an insulating surface has attracted attention. Thin film transistors are used for display devices typified by a liquid crystal TV set. As a semiconductor thin film that can be applied to the thin film transistors, a silicon-based semiconductor material is known, and as another material, an oxide semiconductor attracts attention.
As a material of oxide semiconductor, zinc oxide or a material containing zinc oxide as a component are known. Further, a thin film transistor formed using an amorphous metal oxide (an oxide semiconductor) having an electron carrier concentration less than 1018/cm3 is disclosed (Patent Documents 1 to 3).